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 A187758 Number of ways to write n=x+y (x,y>0) with 2x-3, 2x+3, 6y+1 and 6y+5 all prime 2
 0, 0, 0, 0, 1, 2, 2, 2, 2, 3, 4, 2, 2, 3, 3, 3, 2, 3, 3, 4, 5, 3, 6, 5, 4, 6, 3, 5, 4, 3, 6, 2, 4, 5, 5, 4, 4, 6, 5, 4, 6, 5, 4, 5, 7, 5, 2, 3, 6, 4, 5, 4, 5, 7, 6, 9, 5, 4, 9, 5, 4, 5, 5, 4, 5, 6, 3, 8, 5, 8, 8, 3, 7, 5, 3, 5, 3, 5, 4, 9, 6, 4, 9, 7, 5, 8, 7, 8, 6, 9, 8, 2, 7, 7, 5, 6, 2, 10, 6, 3 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,6 COMMENTS Conjecture: a(n)>0 for all n>4. This has been verified for n up to 10^8. It implies that there are infinitely many cousin primes and also infinitely many sexy primes. LINKS Zhi-Wei Sun, Table of n, a(n) for n = 1..20000 Zhi-Wei Sun, Conjectures involving primes and quadratic forms, arXiv:1211.1588. EXAMPLE a(5)=1 since 5=4+1 with 2*4-3, 2*4+3, 6*1+1 and 6*1+5 all prime. MATHEMATICA a[n_]:=a[n]=Sum[If[PrimeQ[2k-3]==True&&PrimeQ[2k+3]==True&&PrimeQ[6(n-k)+1]==True&&PrimeQ[6(n-k)+5]==True, 1, 0], {k, 1, n-1}] Do[Print[n, " ", a[n]], {n, 1, 100}] CROSSREFS Cf. A023200, A046132, A023201, A002375, A187757, A199920, A219055, A218867, A220455. Sequence in context: A219795 A082602 A216644 * A171931 A221530 A262944 Adjacent sequences:  A187755 A187756 A187757 * A187759 A187760 A187761 KEYWORD nonn AUTHOR Zhi-Wei Sun, Jan 03 2013 STATUS approved

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Last modified February 20 15:10 EST 2019. Contains 320337 sequences. (Running on oeis4.)